A digital subscriber line (DSL for short) technology is a high-speed transmission technology for data transmission by using a telephone line, that is, an unshielded twisted pair (UTP for short), and includes an asymmetrical digital subscriber line (ADSL for short), a very-high-bit-rate digital subscriber line (VDSL for short), a very-high-bit-rate digital subscriber line 2 (VDSL2 for short), a single-pair high-bit-rate digital subscriber line (SHDSL for short), and the like. In various digital subscriber line (xDSL) technologies, xDSL for passband transmission uses a discrete multi-tone (DMT for short) modulation technology to perform modulation and demodulation. A system that provides access for multiple DSL signals is referred to as a DSL access multiplexer (DSLAM for short).
It may be learned from an electromagnetic induction principle that in time division duplex (TDD for short) mode, mutual interference is generated between multiple DSL signals that gain access to the DSLAM, and is referred to as crosstalk (Crosstalk). The crosstalk is classified into near-end crosstalk (NEXT) and far-end crosstalk (FEXT). Energy of the foregoing near-end crosstalk and far-end crosstalk increases with an increase of a frequency band. For a downstream channel, a next-generation copper line broadband access standard (Gfast) of time division multiplexing is used. Because an increasingly wide frequency band is used in the Gfast, the FEXT affects line transmission performance more severely. One method is to use a vectoring technology. That is, DSL signals of multiple users are jointly sent or received at a DSLAM end, FEXT interference is canceled by using a signal processing method, and therefore FEXT interference in each DSL signal is finally eliminated.
Specifically, for downstream, that is, when a signal is sent from the DSLAM end to a client, when joint pre-transmission processing is performed at the DSLAM end, a linear or nonlinear precoding solution may be used. That is, when DSL signals of multiple users are sent at the DSLAM end according to joint channel information between the DSLAM end and multiple clients, linear or nonlinear precoding is performed on the DSL signals, so as to eliminate FEXT interference in each DSL signal. Existing research on nonlinear precoding applied to the DSL mainly focuses on performance of the nonlinear precoding. Generally, a receive end determines a bit loading quantity by using a fixed nonlinear precoding modulus value and an actually measured signal to noise ratio (SNR for short), so that a transmit end may send information about corresponding bits to the receive end according to the bit loading quantity determined by the receive end.
However, it may be found by analyzing and simulating a nonlinear precoding bit loading solution applied to the DSL that the nonlinear precoding modulus value affects the DSLAM end, that is, signal power when the transmit end sends the information about the corresponding bits to the receive end according to the foregoing bit loading quantity, the signal power affects an SNR of a signal received by the receive end, and the SNR affects a bit error rate of the receive end. Therefore, poor system reliability is caused.